Numerous electrochemical processes involve gas-to-liquid or liquid-to-gas transformations. For example, hydrogen-oxygen fuel cells typically utilize the transformation of gaseous oxygen and hydrogen into liquid water at solid-phase, electrically-connected catalysts, like platinum metal.
Many gas-to-liquid or liquid-to-gas processes are most effectively carried out by so-called Gas Diffusion Electrodes (GDEs). At the present time, commercially available GDEs typically comprise fused, porous layers of conductive particles (usually carbon particles) of different size. The outer-most layers typically contain particles of the smallest dimensions, fused together with lesser amounts of hydrophobic PTFE (polytetrafluoroethylene, or Teflon™) binder. The inner-most layers typically contain the largest particles. There may be multiple intermediate layers of intermediary particle size.
The intention of this gradation in particle size within GDEs, from largest in the center to smallest on the outer sides, is to create and control a three-phase solid-liquid-gas boundary within the electrode. This boundary should have the largest possible surface area. The creation of such a boundary is achieved, effectively, by controlling the average pore sizes between the particles, ensuring that the smallest pore sizes are at the edges and the largest are in the center. Since the pores are typically relatively hydrophobic (due to the PTFE binder), the small pore sizes at the edges (e.g. 30 microns pore size) act to hinder and limit the ingress of liquid water into the GDE. That is, water can penetrate only a relatively short distance into the GDE, where the electrochemically active surface area per unit volume, is largest. By contrast, the larger pores in the centre of the GDE (e.g. 150 microns pore size), allow for ready gas transmission at low pressure along the length of the GDE, with the gas then forming a three-way solid-liquid-gas boundary with the liquid water at the edges of the GDE, where the electrochemically active surface area per unit volume is the largest.
Layered porous electrode structures are presently the industry standard for:                (1) conventional free-standing GDEs (for example, of the type used in hydrogen-oxygen PEM fuel cells); and        (2) hybrid GDEs, where a GDE layer has been incorporated within an electrode, typically between a current collector and the gas zone.        
GDEs of this type often display significant technical problems during operation. These largely derive from the difficulty of creating a seamlessly homogeneous particulate bed, with uniform pore sizes and distributions, and uniform hydrophobicity (imparted by the hydrophobic PTFE binder within the GDE). Because of the resulting relative lack of uniformity in the GDE structure, the three-phase solid-liquid-gas boundary created within the GDE may be:                Unstable and fluctuating. The location of the boundary within the GDE may be subject to changing conditions during reaction which cause the boundary to constantly re-distribute itself to new locations within the GDE during operation.        Inhomogeneous. The boundary may be located at widely and unpredictably divergent depths within the GDE as one traverses the length of the GDE.        Inconsistent and ill-defined. At certain points within the GDE, there may be multiple and not a single solid-liquid-gas boundary.        Prone to failure. The boundary may fail at certain points within the GDE during operation, causing a halt to the desired chemical reaction. For example, a common failure mode is that the GDE becomes completely filled with the liquid phase, thereby destroying the three-phase boundary; this is known in the industry as “flooding”. Flooding is a particular problem in fuel cells, such as hydrogen-oxygen fuel cells, that require the feedstock gases to be humidified. Flooding may be caused by water ingress into the gas diffusion electrode via systematic, incremental percolation through the non-homogeneous pores of the electrode, or it may be caused by spontaneous condensation of the water vapour in the feedstock gas stream. In all cases, flooding induces a decline in the voltage output and power generation of such fuel cells.        
Problems of this type are not conducive to optimum operations and may result in uneven, low-yielding, incomplete or incorrect reactions, amongst others.
Conventional 3D Particulate Fixed-Bed Electrodes and GDEs
At the present time, 3D particulate fixed bed electrodes and gas diffusion electrodes (GDEs) are conventionally fabricated by mixing carbon black and PTFE powders and then compressing the solid mixture into a bulk, porous electrode.
The pore size of the resulting structure may be very roughly controlled by managing the particle size of the particulates used. However, it is difficult to achieve a uniform pore size throughout the electrode using this approach because particles, especially “sticky” particles like PTFE, often do not flow evenly and distribute themselves uniformly when compressed. A wide range of pore sizes are therefore typically obtained. It is, moreover, generally not possible to create structures with uniformly small pore sizes, such as 0.05 μm-0.5 μm in size.
The hydrophobicity of the structure is typically controlled by managing the relative quantity of PTFE incorporated into the structure. The PTFE holds the structure together and creates the required porosity. However, its quantity must be carefully controlled so as to impart the electrode with an appropriately intermediate hydrophobicity. An intermediate hydrophobicity is needed to ensure partial, but not complete water ingress. In the case of GDEs, this is needed to thereby create a solid-liquid-gas boundary within the carbon black matrix that makes up the electrode.
This method of constructing 3D particulate fixed bed electrodes and gas diffusion electrodes creates some significant practical problems when operating such electrodes in industrial electrochemical cells, particularly in electro-synthetic and electro-energy (e.g. fuel cell) applications. These problems include the formation of three-way solid-liquid-gas boundaries that are: ill-defined, inconsistent, unstable, fluctuating, inhomogeneous, and prone to failures like flooding.
Problems of this type largely arise from the intrinsic lack of control in the fabrication process, which attempts to create all of the inherent properties of the electrode—including porosity, hydrophobicity, and conductivity—in a single step. Moreover, the fabrication method seeks to simultaneously optimise all of these properties within a single structure. This is often not practically possible since the properties are inter-related, meaning that optimising one may degrade another.
Despite these drawbacks, the approach of combining particulate carbon black and PTFE into a compressed or sintered fixed bed remains the standard method of fabricating GDEs for industrial electrochemistry. This approach is used to fabricate, for example, free-standing GDEs of the type used in hydrogen-oxygen PEM fuel cells. Even where only a GDE component is required within an electrode, the standard method of fabricating that GDE component is to form it as a compressed, porous layer of particulate carbon black and PTFE.
For the above and other reasons, the conventional method of making GDEs and the properties of conventional GDEs, and methods for managing electrochemical reactions occurring therein, are open to improvement.
FIG. 1 (prior art) depicts in a schematic form, a conventional 3D particulate fixed bed electrode or a gas diffusion electrode (GDE) 110, as widely used in industry at present.
In a conventional 3D particulate fixed bed electrode or GDE 110, a conductive element (e.g. carbon particles) is typically combined (using compression/sintering) with a non-conductive, hydrophobic element (e.g. polytetrafluoroethylene (PTFE) Teflon™ particles) and catalyst into a single, fixed-bed structure 110. The fixed-bed structure 110 has intermediate hydrophobicity, good but not the best available conductivity, and a pore structure that is non-uniform and poorly defined over a single region 113. When the 3D particulate fixed bed electrode or GDE 110 is then contacted on one side by a liquid electrolyte and on the other side by a gaseous substance, these physical features bring about the formation of an irregularly-distributed three-phase solid-liquid-gas boundary within the body of the electrode 110, below its outer surface 112 and within single region 113, as illustrated in the magnified view presented in FIG. 1. At the three-phase boundary, electrically connected catalyst (solid phase) is in simultaneous contact with the reactants (in either the liquid or the gas phase) and the products (in the other one of the liquid or gas phase). The solid-liquid-gas boundary within the GDE 110 therefore provides a boundary at which electrochemical liquid-to-gas or gas-to-liquid reactions may be facilitated by, for example, the application of a particular electrical voltage. The macroscopic width of the three-phase solid-liquid-gas boundary is comparable or similar in dimension to the width of the conventional GDE. The thickness of the three-phase solid-liquid-gas boundary in a conventional GDE is typically in the range of from 0.4 mm to 0.8 mm in fuel cell GDEs up to, higher thicknesses, such as several millimeters, in industrial electrochemical GDEs.
Overpressure and Flooding
The phenomenon of flooding described above is often caused by ingress of liquid electrolyte, for example water, into the gas diffusion electrode when the liquid electrolyte is subject to any sort of external pressure. For example, in an industrial electrolytic cell of 1 meter height, water at the bottom of the cell is pressurised at 0.1 bar due to the hydraulic head of water. If a GDE were used at this depth, the GDE would typically be immediately flooded by water ingress because modern-day GDEs have very low “wetting pressures” (also known as the “water entry pressure”), that are typically less than 0.1 bar (although GDEs with wetting pressures of 0.2 bar have recently been reported in WO2013037902). GDEs are, additionally, relatively expensive.
This is a particular problem in industrial electrochemical cells in which it is highly beneficial to apply a gas, for example such as oxygen or hydrogen, to an electrode, via the use of a GDE at that electrode. Many industrial electrochemical cells are large, employing water electrolyte with a depth greater than 1 meter in the cell. If a GDE with wetting pressure less than 0.1 bar is used, then the cell will leak from its electrolyte chamber unless additional means are applied to balance the pressure differential between the liquid and the gas side of the GDE.
Companies have therefore gone to great lengths to try to operate GDEs at or near 0.1 bar. For example, WO 2003035939 describes a method for segmenting the water head so that a depth of more than 1 meter can be used but no part of the GDE feels more than 0.1 bar trans-electrode pressure.
In other industrial electrochemical cells, electrolyte is routinely pumped around the cell. Unless expensive pressure-compensation equipment is installed to scrupulously avoid pressure differentials, such pumping actions may readily generate local increases in the liquid pressure of 0.1 bar or more, thereby causing the cell to leak from its electrolyte chamber if a GDE was used as one of the electrodes.
The counterpart to the problem of flooding relates to the development of excess pressure on the gaseous side of conventional GDEs. If the gas pressure in a conventional GDE becomes even a little higher than the liquid pressure, then excess gas may pass through the GDE, exiting as bubbles at the liquid-facing side of the GDE. The formation of bubbles in this way is generally deleterious to the performance of the cell in that bubbles typically: (i) “mask” the electrode surface, causing a decline in the rate of reaction, and (ii) increase the ionic resistance of the electrolyte solution, inducing unnecessary energy consumption.
These problems related to pressure equalisation at the gas-liquid interface in GDEs are compounded by the fact that many industrial electrochemical cells operate most effectively when the liquid electrolyte is pressurised to, for example, several bars of pressure. As it can be extremely difficult to maintain pressure equalisation at a GDE, solid-state electrodes, such as dimensionally-stable electrodes, must instead be used. Such electrodes typically generate bubbles of the product gases, so that, the gas-liquid interface for such electrodes involves the surface of the gas bubbles present in the liquid electrolyte at the electrode face. The gas within such bubbles must necessarily and automatically be at the same pressure as the surrounding liquid electrolyte, in order for the bubble to resist collapse, or to avoid uncontrolled expansion. Under these circumstances, pressure equalisation is not a challenge and the problem is thereby avoided.
The technical problems associated with commercially available GDEs, along with their high cost and other factors, mean that it is generally commercially and technically unviable to use GDEs in many present-day industrial electrochemical gas-to-liquid or liquid-to-gas processes.
This is demonstrated by the very extensive efforts that have been made over the years seeking to develop GDEs and pressure-equalising apparatuses to deal with these challenges in the case of the chlor-alkali process. The chlor-alkali process is one of the most widely used electrochemical processes in the world. Numerous patent applications have described approaches seeking to overcome the problem of pressure equalisation in the chlor-alkali process. For example, Patent Publication Nos. WO 2003035939, WO 2003042430, and, more recently, WO 2013037902, have described pressure-equalising apparatuses and/or fabrication techniques to create Gas Diffusion Electrodes able to avoid leaking up to 0.2 bar liquid overpressure.
In summary, in order to realise the benefits that GDEs may confer upon electrochemical gas-to-liquid and liquid-to-gas processes, new electrochemical cells, GDEs and/or methods to manage electrochemical reactions or pressure differentials at the liquid-gas interface of GDEs are needed. Preferably, an improved Gas Diffusion Electrode should be relatively inexpensive, robust and/or mechanically strong, and have a relatively high wetting pressure.
The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.